SySt are believed to spend most of their time in the accreting-only phase,
quietly accumulating material on the surface of the WD. When enough has
been piled up, nuclear burning ignites. If the accreted matter is electron
degenerate, the burning proceeds explosively. It quickly reaches the Fermi
temperature (a matter of minutes, [1]}) and ejects most of
the accreted envelope at high velocity (thousands of km s\(^{-1}\) ).
Radio-interferometry has nicely resolved the expansion of ejecta following,
for ex., the 2006 outburst of RS Oph [2]} and that of
2010 for V407 Cyg [3]}, the latter being also the
first nova event ever detected in GeV \(\gamma \) -rays by the Fermi satellite
[4]}. The resulting nova outburst can repeat on
time-scales as short as years/decades if the WD mass is close to the
Chandrasekhar limit, like in the case for T CrB, RS Oph, V407 Cyg or V3890
Sgr. If the matter accreted on the WD is instead not electron-degenerate,
the nuclear burning proceeds in thermal equilibrium, and no mass is ejected.
The event takes a few years to reach peak brightness
[5]}, and requires many decades to a few centuries to
burn the accreted envelope and let the system return to low luminosity.
Some of the best examples of this type are AG Peg, HM Sge, V1016 Cyg and
V4368 Sgr.
